1. Field of the Invention
The present invention relates to a manufacturing method of a press product and a press forming apparatus capable of obtaining a product by bending a workpiece.
2. Related Art
For example, when a corner part is formed on a metal plate by a drawing process or a bending process, a press forming of the metal plate is performed by applying a predetermined pressing force on the metal plate to bend the metal plate by a predetermined angle. As a result, a product having the corner part and its surrounding parts is obtained. However, if the product is released from the pressing force, the corner part and its surrounding parts bounce in a direction opposite to an applying direction of the pressing force. That is, so-called spring-back is occurred. Especially, the spring-back is remarkable in a high tensile strength material such as a high tensile steel plate.
Of course, the forming die for performing the press forming process is set so that an angle of the corner part of the metal plate after the spring-back occurs corresponds to a predetermined angle. That is, the forming die is designed in consideration of the spring-back so that a product having a desired shape is obtained.
However, even if the forming die thus designed is used to perform the press forming process, there is often the case that a real quantity of the spring-back is different from an estimated quantity of the spring-back. In this case, it is necessary to amend the forming die.
As such, dimensions or the pressing force of the forming die for the press forming process leads to a final setting by repeating a test of the press forming, a correction of the forming die based on the real quantity of the spring-back and retest of the press forming. On this account, the number of the process leading up to the final setting is increased and a long manufacturing time is required.
Accordingly, it is required to reduce the spring-back as far as possible. In this case, since the non-uniformity in the amount of the spring-back is also decreased, it is easy to estimate the amount of the spring-back. As a result, it is expected that the number of the process leading up to the final setting of the forming die is reduced and the manufacturing time is short.
JP-A-08-215759 discloses a technique for locally upsetting the workpiece in order to reduce the spring-back. Specifically, a forming die having a protrusion presses a corner part of the workpiece and thus the corner part is plastically deformed. As a result, a golf club head of which thickness is locally thin is obtained.
In the technique disclosed in JP-A-08-215759, the corner part of the workpiece is locally pressed by the protrusion while the workpiece is constrained in the forming die. Therefore, the workpiece is clinched by the protrusion and thus the material is difficult to flow during the press forming process. For this reason, there is a risk that the workpiece is broken. Further, since the thickness of the corner part is locally thin, the strength of the corner part is degraded. Consequently, it is difficult to adopt such a press-product as a high strength structural material such as a vehicle panel, for example.